1. Field of the Invention
The invention relates to the use of water-insoluble polyoxyalkylene-polysiloxane block copolymers for defoaming aqueous media.
In numerous industrial processes, aqueous formulations use surface-active substances in order, for example, to emulsify water-insoluble substances or to reduce the interfacial tension and so achieve better wetting characteristics. An unwanted accompanying effect of these surface-active substances, however, is that air introduced during the preparation and processing of such water-based formulations is stabilized as foam. Examples of processes in which such problems occur include papermaking, wastewater treatment, emulsion polymerization, and the manufacture and application of water-dilutable coating systems.
Foam of this kind, produced in the course of stirring or dispersing operations during manufacture or filling, prolongs production times and reduces the effective capacity of the production plant Foam is disruptive to the application of coating materials since it leads to unwanted surface defects. As a result, the use of antifoams or defoamers in virtually all water-based systems is widespread and often indispensable.
In the past, a large number of formulations have been described which envisage the use of, for example, silicone oils, organo modified siloxanes, hydrophobic polyoxyalkylenes, mineral oils, natural oils and other hydrophobic liquids as defoaming substances. Frequently, the above-mentioned substances are used in combination with hydrophobic solids, such as silicas, metal stearates or fatty acid amides, for example, which often intensify the foam inhibiting or defoaming effect.
In accordance with the present state of knowledge, it is critical to the defoaming action of a defoamer that it is able to penetrate into the foam lamellae and so to weaken them until they collapse (cf. K. Koezo, J. K. Koezone, D. T. Wasan, J. Colloid Interface Sci. 166, 225-238 (1994)). To achieve this requires a controlled incompatibility (hydrophobicity) with the aqueous phase to be defoamed: if a defoamer is too compatible (hydrophilic), it will not be able to be very effective since it does not penetrate preferentially into the foam lamella. If the degree of incompatibility is too great, then defoaming is generally very good but there are frequently the unwanted side effects of surface defects, deterioration in wetting characteristics, and separation phenomena.
Consequently, the search for a suitable defoamer is always a search for the correct incompatibility/compatibility balance for the system to be defoamed, with the aim of getting as close as possible to the target hydropbobicity/hydrophilicity equilibrium
2. Description of the Related Art
In the past, polyoxyalkylene-polysiloxane block copolymers have proven suitable for allowing this equilibrium to be established in a particularly variable manner, as is described, inter alia, in U.S. Pat. No. 3,763,021.
When polyoxyalkylene-polysiloxane block copolymers are used, advantage is taken of the fact that the polysiloxane blocks can be modified in a targeted manner with polyoxyalkylene units which, by virtue of their composition from hydrophilic and hydrophobic polyoxyalkylene units, can be adapted, in interplay with the polysiloxane block, to the abovementioned desired hydrophobicity/hydrophilicity equilibrium.
The prior art, an example of which that may be mentioned here is DE-C-1 012 602, includes polyoxyalkylene-polysiloxane block copolymers in which the polyoxyalkylene block is defined as (CnH2nO)x. The index n is an integer from 2 to 4, it being essential that these polyoxyalkylene blocks always include xe2x80x94CH2xe2x80x94CHRxe2x80x94Oxe2x80x94 groups as a common structural element, R being a hydrogen atom, a methyl group or an ethyl group. Such polyoxyalkylene blocks are prepared by ionic addition polymerization of oxirane derivatives, such as ethylene oxide, propylene oxide and butylene oxide. The basic structure of the properties-determining polyoxyalkylene chain is distinguished, however, by generally the combination of only two different polyoxyalkylene units in a polyoxyalkylene chain. Typically, polyoxyethylenes and polyoxypropylenes are used in the polyoxyalkylene-polysiloxane block copolymers of the prior art.
By varying the oxyalkylene units, and in particular by modifying their respective proportions and their sequence in the polymer (random and/or in blocks), a large number of polyoxyalkylene-polysiloxane block copolymers are obtainable.
Because of the restriction to only two polyoxyalkylene units, however, in many cases it has not been possible to date to find the optimum incompatibility/compatibility balance required for certain defoamers, and especially not when their random distribution is permitted.
It is therefore an object of the present invention to provide particularly suitable polyoxyalkylene-polysiloxane block copolymers for defoaming aqueous media, said copolymers making it possible to establish the above-described desired incompatibility/compatibility balance in a targeted manner better than has been possible to date.
This object is surprisingly achieved by the use of water-insoluble polyoxyalkylene-polysiloxane block copolymers of the general average formula I: 
where
R1 and/or R7 are R2 or xe2x80x94[R4]wxe2x80x94[R5]xxe2x80x94[R6]xe2x80x94R8,
R2 and R3 are identical or different alkyl radicals, preferably having 1 to 24 carbon atoms or are unsubstituted or substituted aryl radicals, preferably having up to 24 carbon atoms, with phenyl especially preferred,
R4 is a divalent radical of the formula xe2x80x94Oxe2x80x94, xe2x80x94NHxe2x80x94, xe2x80x94NR2xe2x80x94 or xe2x80x94Sxe2x80x94 or of the formula
xe2x80x94[Oxe2x80x94Si(CH3)2]uxe2x80x94,
xe2x80x83where
u is from 2 to 200,
R5 is identical or different alkylene radicals, preferably having 1 to 24 carbon atoms, or is
xe2x80x94CnH2n-fR21xe2x80x94Oxe2x80x94CmH2m-gR2gxe2x80x94,
xe2x80x83where
f is from 0 to 12,
g is from 0 to 12,
n is from 1 to 18,
m is from 1 to 18,
R6 is identical or different radicals of the general formula
xe2x80x94Oxe2x80x94(C2H4O)a(CH2xe2x80x94CH(CH3)O)b(CH2xe2x80x94CR9R10O)cxe2x80x94,
xe2x80x83where
R9 and R10 are a methyl radical or
R9 is a hydrogen atom and R10 is an ethyl radical,
xe2x80x83but where, taking into account all radicals R6, the proportion of C2H4O groups among the total of all radicals R6does not exceed 40% by weight,
a is from 2 to 30,
b is from 2 to 50,
c is from 2 to 40, and
the sum (a+b+c) is from 6 to 100
and the sequence of the individual polyoxyalkylene segments xe2x80x94(C2H4O)axe2x80x94 and xe2x80x94(CH2xe2x80x94CH(CH3)O)bxe2x80x94 and xe2x80x94(CH2xe2x80x94CR9R100)cxe2x80x94 can be as desired and, in particular, embraces block copolymers, such as random polymers and combinations thereof,
R8 is a hydrogen, a substituted or unsubstituted alkyl radical, preferably having 1 to 6 carbon atoms, or an acyl radical,
v is from 3 to 200,
w is 0 or 1,
x is 0 or 1,
y is from 0 to 200,
and, if y is 0, R1 and/or R7 are/is xe2x80x94[R4]wxe2x80x94[R5]xxe2x80x94[R6]xe2x80x94R8 for defoaming aqueous media.
In many cases, polyoxyalkylene-polysiloxane block copolymers which have proven highly suitable are those obtained by means of combinations of two or more polyoxyalkylene units with one or more polysiloxane copolymers. In that case, the distinction in the case of the polyoxyalkylene copolymers can be a distinction in relation either to the relative proportion of the units used for the preparation of this copolymer or to the molecular weight of the whole polyoxyalkylene unit, or else can lie in its functionality.
Different polysiloxane copolymers can be distinguished in accordance with both their molecular mass and their degree of branching or the number and/or relative position of the reactive groups used which are available for linkage.
The mode of action of the compounds of the invention is expressly not that they bring about particularly good compatibilization between constituents of a mixture that differ in polarity. Rather, it is the case that they themselves are just compatible enough not to induce any unwanted side effects such as surface defects, deteriorations in the wetting characteristics, or separation phenomena, but without bringing about the compatibilization of other constituents.
For the present invention it is essential that the polyoxyalkylene-polysiloxane block copolymers of the invention are not water-soluble. This means that a 1:1 mixture (% by weight) with water leads to a nonhomogeneous mixture of the two phases and that no dissolving of the polyoxyalkylene-polysiloxane block copolymers of the invention takes place. However, small fractions of free polyoxyalkylenes may be present in the polyoxyalkylene-polysiloxane block copolymers as a result of their preparation as set out below. Since the preparation of these copolymers essentially obeys the laws of statistics, it is also possible for particularly oxyethylene-rich polyoxyalkylenes to occur, which in turn may be soluble in water. These fractions, however, are very small and in the polyoxyalkylene-polysiloxane block copolymers of the invention are always less than 10% by weight.
The polyoxyalkylene-polysiloxane block copolymers of the invention can be prepared in accordance with customary methods as in the prior art. As the skilled worker is well aware, such methods include in particular the hydrosilylation reaction of additionally olefinically unsaturated polyoxyalkylenes with hydropolysiloxanes, as described for example in U.S. Pat. No. 3,402,192, or the nucleophilic substitution of polyoxyalkylenes with nucleophilic groups on polysiloxanes containing at least one silicon atom substituted by an electronegative group (cf., for example, DE-B-11 65 028). Processes of this kind and the preparation of appropriate polysiloxanes are further described, moreover, for example, in W. Noll xe2x80x9cChemie und Technologie der Siliconexe2x80x9d, Verlag Chemie, Weinheim, 1968.
In accordance with the prior art, the polyoxyalkylenes required for this purpose can be obtained by ionic polymerization of ethylene oxide, propylene oxide, n-butylene oxide, and isobutylene oxide.